Quinoline derivatives useful for protecting cultivated plants from the phytotoxic action of herbicides (i.e. useful as “safeners”) are known, for example, from EP-A-0 094 349, U.S. Pat. No. 5,102,445 and U.S. Pat. No. 5,441,922.
According to U.S. Pat. No. 5,102,445, such quinoline derivatives, in particular an alkyl (8-quinolinoxy)-acetate or an alkyl 2-(8-quinolinoxy)-propionate or a ring-substituted derivative of either of these, can be prepared inter alia by reacting 8-hydroxyquinoline or a ring-substituted derivative thereof with a haloacetic acid derivative (such as an alkyl haloacetate, e.g. an alkyl chloroacetate) or a 2-halo-propionic acid derivative (such as an alkyl 2-halo-propionate, e.g. methyl 2-bromo-propionate), typically in the presence of a base (e.g. potassium carbonate) in an inert solvent (e.g. butan-2-one) at elevated temperature, preferably in the presence of a catalytic amount of alkali metal iodide. See columns 2, 17, 21, 22 and 23-24 (Example 1) in U.S. Pat. No. 5,102,445. The yields obtained are often not very satisfactory, especially for the large-scale preparation of those compounds. Furthermore, undesirable by-products, e.g. alcohols, which can significantly reduce product quality, can be formed in that process.
Cloquintocet-mexyl is commercially used as a safener of herbicides (specifically, the grass-active herbicides clodinafop-propargyl or pinoxaden, or the herbicide pyroxsulam, or the herbicide flucarbazone or a salt (e.g. sodium salt) thereof) in small grain cereal crops such as wheat (see, e.g., The Pesticide Manual, 15th edition, 2009, British Crop Production Council, entry 174, pages 226-227). It accelerates the detoxification process of clodinafop-propargyl in cereals e.g. wheat (Kreuz et al., Z. Naturforsch., 1991, 46c, pp. 901-905). The IUPAC chemical name of cloquintocet-mexyl is 1-methylhexyl (5-chloroquinolin-8-yloxy)acetate, and its chemical structure is:

The free acid (ester-free) derivative of cloquintocet-mexyl, (5-chloroquinolin-8-yloxy)acetic acid (“cloquintocet acid”), is also thought to be useful as a herbicide safener, e.g. as a safener for clodinafop-propargyl, pinoxaden, flucarbazone or a salt (e.g. sodium salt) thereof, or, in particular, pyroxsulam. Cloquintocet acid has the following structure:

WO 02/00625 A discloses a process for the preparation of a compound of formula (I)
wherein R1 is hydrogen or chlorine, and R2 is hydrogen, C1-C8 alkyl, or C1-C8 alkyl substituted by C1-C6 alkoxy or by C3-C6 alkenyloxy, which process comprises a) introducing the major portion of the amount to be reacted of a compound of formula (II)
into a solvent mixture comprising at least one organic solvent capable of forming an azeotrope with water, and at least one aprotic-dipolar solvent; b) metering in an aqueous strong base (preferably an alkali metal hydroxide, e.g. NaOH or KOH, or an alkaline earth metal hydroxide) in an amount equivalent to that major portion of the total amount of the compound of formula (II); c) adding the remaining portion of the amount to be reacted of the compound of formula (II); d) adding a weak base (preferably an alkali metal carbonate, e.g. Na2CO3 or K2CO3, or an alkaline earth metal carbonate) in an amount that is at least equivalent to that remaining portion; e) removing the water from the reaction mixture by azeotropic distillation; f) adding a compound of formula (III)
wherein R2 is as defined for formula (I); and g) isolating the resulting compound of formula (I) from the reaction mixture.
This WO 02/00625 A process is good, but is not perfect for preparation of compounds of formula (I) wherein R2 is hydrogen, as the chloroacetic acid starting material (III, R2═H) forms a salt under the basic reaction conditions used.
The aim of the present invention is accordingly to provide a process for the preparation of non-ester quinoline carboxylic acids, specifically (5-chloroquinolin-8-yloxy)acetic acid (“cloquintocet acid”, which is useful as a herbicide safener) or (quinolin-8-yloxy)acetic acid, that is distinguished by high yields and/or good product quality and/or that avoids one or more possible disadvantages of alternative processes.